The invention relates to an attachment device for a cryogenic satellite tank.
The service life of a satellite equipped with a cryogenic tank is to a large extent determined by heat transfer or thermal flow to the cryogenic tank, said flow emanating from the warmer satellite by the attachment device of the tank to the tank. This thermal flow results in an increase in pressure in the tank, which is compensated by partial discharge of the tank""s contents.
As a result of the thermal flow via the attachment device, the achievable cooling temperatures are limited both for active and passive cooling, and the coolant consumption is increased as a result of the thermal flow.
The substantial mechanical loads occurring during launch of the satellite mitigate against adequate reduction in material cross sections of the attachment device, so that optimized design of the attachment device from the point of view of material cross sections cannot adequately reduce the thermal flow via the attachment device, even if materials having low thermal conductivity are used.
EP 0 584 697 B1 discloses a deice for holding a storage container by means of tensile stress within an object, particularly in space technology. This device ensures a stable connection between the storage container and the object during the launch phase, and at comparatively low mechanical loads, ensures that the holding device between the storage container and the object is of low thermal conductivity. This is achieved by use of a shape memory alloy (SMA).
It is an object of the invention to provide an attachment device for a cryogenic satellite tank which has reduced thermal conductivity and provides adequate rigidity and strength, while being simple in design and easy to produce from a technical point of view, is not prone to malfunction and thus has low maintenance requirements.
The thermal conductivity between two objects engaged against each other is substantially determined by their surface pressure and contact area. This experience is taken into account in many solutions proposed by the invention. The attachment devices according to the invention advantageously use shape memory alloy elements, which, following the rocket launch, in the cold space environment, automatically expand significantly and reduce the surface pressure at the respective connection points and partially break the contact therebetween. Consequently, the thermal flow via these contact points is reduced or completely eliminated. The resulting reduction in rigidity of the attachment device can be adequate for use in the space environment if the connection points have been suitably designed. Alternatively, apart from the connection points, controlled by SMA elements, the attachment device can comprise additional continuously rigid attachment means which ensures adequate rigidity and maintenance of the tank alignment.
The decisive characteristic of SMA elements is the thermoplastic transformation from the high-temperature austenite state to the low-temperature martensite state. When such an element cools down, below a limit temperature TMS, the formation of martensitic phases commences which is completed with further cooing at a temperature TMR, at which attainment of the complete martensitic state is obtained. When heated from the martensitic state, above a temperature TAR, the austenitic phase is formed. This transformation to the austenitic phase is fully complete when the temperature TAR is reached.
Thermomechanical training of the SMA elements results in a cyclic change in length both during heating and during cooling without any associated load: the so-called two-way effect. This training is necessary to achieve an additional change in length beyond the mechanical balance which is necessary for extension according to the invention, of the connection points of the attachment device. The entire achievable change in length of the SMA elements after carrying out thermomechanical training ranges from approximately 1.5% to 4%.
Preferably, the SMA elements are made from a NiTi-based alloy, but SMA alloys with a different composition can be used as long as they satisfy the transformation temperatures and changes in length according to the invention. In the case of NiTi-based alloys, for example, quaternary alloys such as NiTiCuFe or NiTiCuCr are used.
The composition of these alloys is selected such that the transformation temperature for completion of the austenite formation is TAR less than 0 degree C., so that the SMA elements of the attachment device for the ambient temperatures during the launch phase, when the largest acceleration forces act upon the attachment device, are safely fully within the austenitic state. The final temperature for martensite formation TMR is set to a value above the temperature in space experienced by the attachment device, such temperature depending on the respective satellite emission; typically between 2xc2x0 K and 220xc2x0 K.
In the austenitic state, the SMA material according to the invention has a tensile strength of 700 MPa to 980 MPa and a modulous of elasticity of 80 GPa to 100 GPa and provides good mechanical properties so that if it is suitably dimensioned, it can easily resist and transmit the forces acting on the attachment device during rocket launch. In the martensitic state the material is relatively soft.